Semiconductor Device Including a Heat Sink Structure

ABSTRACT

A semiconductor device includes a drift structure formed in a semiconductor body. The drift structure forms a first pn junction with a body zone of a transistor cell. A gate structure extends from a first surface of the semiconductor body into the drift structure. A heat sink structure extends from the first surface into the drift structure. A thermal conductivity of the heat sink structure is greater than a thermal conductivity of the gate structure and/or a thermal capacity of the heat sink structure is greater than a thermal capacity of the gate structure.

PRIORITY CLAIM

This application claims priority to German Patent Application No. 102015 122 804.1 filed on 23 Dec. 2015, the content of said applicationincorporated herein by reference in its entirety.

BACKGROUND

Power semiconductor devices transform to some degree electric energyinto thermal energy. In case a short-circuit occurs or in case the powersemiconductor device turns off in response to an over-current condition,the dissipated thermal energy may significantly exceed the thermalenergy dissipated during normal operation mode. Designing asemiconductor body of the semiconductor device to reliably sustainshort-term thermal stress typically adversely impacts devicecharacteristics. Conventionally, a thick copper metallization isthermally coupled to the semiconductor body and dissipates the thermalenergy generated in the semiconductor body.

There is a need for semiconductor devices with high thermal resilience.

SUMMARY

According to an embodiment a semiconductor device includes a driftstructure which is formed in a semiconductor body and which forms afirst pn junction with a body zone of a transistor cell. A gatestructure extends from a first surface of the semiconductor body intothe drift structure. A heat sink structure extends from the firstsurface into the drift structure. A thermal conductivity of the heatsink structure is greater than a thermal conductivity of the gatestructure and/or a thermal capacity of the heat sink structure isgreater than a thermal capacity of the gate structure.

According to another embodiment a semiconductor device includes asemiconductor body that includes semiconducting portions of a transistorcell, A source contact structure directly adjoins at least a source zoneof the transistor cell at a front side of the semiconductor body. A heatsink feature extends from a rear side into the semiconductor body. Atleast one of (i) a thermal conductivity of the heat sink feature isgreater than a thermal conductivity of the semiconductor body and (ii) athermal capacity of the heat sink structure is greater than a thermalcapacity of the semiconductor body.

Those skilled in the art will recognize additional features andadvantages upon reading the following detailed description and onviewing the accompanying drawings.

BRIEF DESCRIPTION OF THE DRAWINGS

The accompanying drawings are included to provide a furtherunderstanding of the invention and are incorporated in and constitute apart of this specification. The drawings illustrate the embodiments ofthe present invention and together with the description serve to explainprinciples of the invention. Other embodiments of the invention andintended advantages will be readily appreciated as they become betterunderstood by reference to the following detailed description.

FIG. 1A is a schematic vertical cross-sectional view of portions of asemiconductor device including gate structures and heat sink structuresextending from the first surface into a semiconductor body according toan embodiment.

FIG. 1B is a schematic cross-sectional view of portions of asemiconductor device according to an embodiment including field platestructures in addition to gate structures and heat sink structures.

FIG. 2A is a schematic vertical cross-sectional view of a portion of asemiconductor device according to an embodiment related to heat sinkstructures with high thermal capacitance and electrically insulated froma load electrode.

FIG. 2B is a schematic vertical cross-sectional view of a portion of asemiconductor device according to an embodiment related to heat sinkstructures with high thermal capacitance and electrically insulated froma load electrode in combination with field plate structures.

FIG. 2C is a schematic vertical cross-sectional view of a portion of asemiconductor device according to an embodiment related to dielectricheat sink structures with high thermal conductivity and thermallyconnected to a load electrode in combination with field platestructures.

FIG. 2D is a schematic vertical cross-sectional view of a portion of asemiconductor device according to an embodiment related to a combinationof field plate structures and heat sink structures with high thermal andelectric conductivity, wherein the heat sink structures are electricallyconnected to a load electrode.

FIG. 2E is a schematic vertical cross-sectional view of a portion of asemiconductor device according to an embodiment related to heat sinkstructures with high thermal and electric conductivity, wherein the heatsink structures are effective as field plate structures.

FIG. 3A is a schematic vertical cross-sectional view of a portion of asemiconductor device with a homogeneous heat sink structure based on amaterial with high thermal capacitance according to an embodiment.

FIG. 3B is a schematic vertical cross-sectional view of a portion of asemiconductor device with a heat sink structure based on a PCM (phasechange material) and a barrier layer according to a further embodiment.

FIG. 30 is a schematic vertical cross-sectional view of a portion of asemiconductor device with a heat sink structure based on a partial fillwith PCM according to a further embodiment.

FIG. 3D is a schematic vertical cross-sectional view of a portion of asemiconductor device with a heat sink structure based on a PCM with airpockets according to a further embodiment.

FIG. 3E is a schematic vertical cross-sectional view of a portion of asemiconductor device with heat sink structures including a thermallyhigh-conductive dielectric layer.

FIG. 3F is a schematic vertical cross-sectional view of a portion of asemiconductor device with heat sink structures including thermallyhigh-conductive structures along an interface to a semiconductor bodyaccording to an embodiment referring to carbon nanotubes.

FIG. 3G is a schematic vertical cross-sectional view of a portion of asemiconductor device with heat sink structures including thermallyhigh-conductive structures along an interface to a semiconductor bodyaccording to an embodiment referring to diamond crystallites.

FIG. 3H is a schematic vertical cross-sectional view of a portion of asemiconductor device with PCM-based heat sink structures includingthermally high-conductive inclusions according to an embodimentreferring to diamond crystallites.

FIG. 3I is a schematic vertical cross-sectional view of a portion of asemiconductor device with PCM-based heat sink structures includingthermally high-conductive inclusions according to an embodimentreferring to graphene leaves.

FIG. 4A is a schematic vertical cross-sectional view of a portion of asemiconductor device releasing thermal energy outside of a semiconductorbody according to an embodiment referring to heat sink structures withthermally high-conductive dielectric portions.

FIG. 4B is a schematic vertical cross-sectional view of a portion of asemiconductor device releasing thermal energy outside of a semiconductorbody according to an embodiment referring to heat sink structures withdielectric portions thinner than a gate dielectric.

FIG. 5A is a schematic vertical cross-sectional view of a portion of ann-FET according to an embodiment.

FIG. 5B is a schematic vertical cross-sectional view of a portion of areverse blocking n-channel IGBT according to an embodiment.

FIG. 50 is a schematic vertical cross-sectional view of a portion of areverse conducting n-channel IGBT according to an embodiment.

FIG. 6A is a schematic vertical cross-sectional view of a portion of asemiconductor device releasing thermal energy through heat sink featuresat a rear side according to an embodiment.

FIG. 6B is a schematic vertical cross-sectional view of a portion of asemiconductor device releasing thermal energy through heat sink featuresat a rear side according to an embodiment referring to trench gatestructures.

FIG. 7A is a schematic perspective view of a portion of a semiconductordevice according to an embodiment with stripe-shaped heat sink featuresconnected to a load electrode at a rear side.

FIG. 7B is a schematic vertical cross-sectional view of thesemiconductor device portion of FIG. 7A along line B-B.

FIG. 7C is a schematic vertical cross-sectional view of thesemiconductor device portion of FIG. 7A along line C-C.

FIG. 8A is a schematic perspective view of a portion of a semiconductordevice according to an embodiment with stripe-shaped heat sink featuresconnected to a load electrode at a rear side and extending up to a frontside.

FIG. 8B is a schematic vertical cross-sectional view of thesemiconductor device portion of FIG. 8A along line B-B.

FIG. 8C is a schematic vertical cross-sectional view of thesemiconductor device portion of FIG. 8A along line C-C.

FIG. 9A is a schematic perspective view of a portion of a semiconductordevice according to an embodiment with column-like heat sink featuresconnected to a load electrode at a rear side.

FIG. 9B is a schematic vertical cross-sectional view of thesemiconductor device portion of FIG. 9A along line B-B.

FIG. 9C is a schematic vertical cross-sectional view of thesemiconductor device portion of FIG. 9A along line C-C.

FIG. 10A is a schematic perspective view of a portion of a semiconductordevice according to an embodiment with stripe-shaped heat sink featuresformed at a front side.

FIG. 10B is a schematic vertical cross-sectional view of thesemiconductor device portion of FIG. 10A along line B-B.

FIG. 10C is a schematic vertical cross-sectional view of thesemiconductor device portion of FIG. 10A along line C-C.

FIG. 11A is a schematic cross-sectional view of a portion of asemiconductor device according to an embodiment with not-evenlydistributed heat sink features.

FIG. 11B is a schematic vertical cross-sectional view of thesemiconductor device portion of FIG. 11A along line B-B.

FIG. 11C is a schematic vertical cross-sectional view of thesemiconductor device portion of FIG. 11A along line C-C.

DETAILED DESCRIPTION

In the following detailed description, reference is made to theaccompanying drawings, which form a part hereof and in which are shownby way of illustrations specific embodiments in which the invention maybe practiced. It is to be understood that other embodiments may beutilized and structural or logical changes may be made without departingfrom the scope of the present invention. For example, featuresillustrated or described for one embodiment can be used on or inconjunction with other embodiments to yield yet a further embodiment. Itis intended that the present invention includes such modifications andvariations. The examples are described using specific language, whichshould not be construed as limiting the scope of the appending claims.The drawings are not scaled and are for illustrative purposes only. Forclarity, the same elements have been designated by correspondingreferences in the different drawings if not stated otherwise.

The terms “having,” “containing,” “including,” “comprising” and the likeare open, and the terms indicate the presence of stated structures,elements or features but do not preclude additional elements orfeatures. The articles “a,” “an” and “the” are intended to include theplural as well as the singular, unless the context clearly indicatesotherwise.

The term “electrically connected” describes a permanent low-ohmicconnection between electrically connected elements, for example a directcontact between the concerned elements or a low-ohmic connection via ametal and/or highly doped semiconductor. The term “electrically coupled”includes that one or more intervening element(s) adapted for signaltransmission may be provided between the electrically coupled elements,for example elements that are controllable to temporarily provide alow-ohmic connection in a first state and a high-ohmic electricdecoupling in a second state.

The Figures illustrate relative doping concentrations by indicating “−”or “+” next to the doping type “n” or “p”. For example, “n-” means adoping concentration which is lower than the doping concentration of an“n”-doping region while an “n+”-doping region has a higher dopingconcentration than an “n”-doping region. Doping regions of the samerelative doping concentration do not necessarily have the same absolutedoping concentration. For example, two different “n”-doping regions mayhave the same or different absolute doping concentrations.

FIG. 1A shows a portion of a semiconductor device 500, which may be, forexample, an MGD (MOS gated diode), an IGFET (insulated gate field effecttransistor), e.g., a MOSFET (metal oxide semiconductor FET) in the usualmeaning including FETs with metal gate and FETs with semiconductor gate,an IGBT (insulated gate bipolar transistor), e.g., a reverse blockingIGBT or a reverse conducting IGBT, or a device including furtherelectronic circuits in addition to an MGD, IGFET, or IGBT functionality.

The semiconductor device 500 includes transistor cells TC.Semiconducting portions of the transistor cells TC are formed in asemiconductor body 100, which is formed from crystalline semiconductormaterial such as silicon (Si), germanium (Ge), silicon germanium (SiGe),silicon carbide (SiC) or an A_(III)B_(V) semiconductor.

A distance between a first surface 101 at a front side of thesemiconductor body 100 and an opposite second surface at a rear side maybe between 20 μm and several 100 μm. Directions parallel to the firstsurface 101 are horizontal directions and a direction perpendicular tothe first surface 101 is a vertical direction.

The transistor cells TC may be IGFET cells with gate structures 150extending from the first surface 101 into the semiconductor body 100.The gate structures 150 may be stripe-shaped with a first horizontalextension exceeding at least ten times a second horizontal extensionorthogonal to the first horizontal extension or may be dot-shaped withboth horizontal extensions within the same order of magnitude, e.g.,approximately equal.

The gate structures 150 may include a conductive gate electrode 155 anda gate dielectric 151 separating the gate electrode 155 from thesemiconductor body 100. According to other embodiments, the transistorcells TC may be JFET (junction field effect transistor) cells with aconductive gate electrode directly adjoining to the semiconductor body100. The gate electrode 155 may be electrically connected to a controlelectrode 330 which may be formed at the front side and which may formor may be electrically connected or coupled to a gate terminal G.

The transistor cell TO includes a body zone 115 formed in an active mesa181 of the semiconductor body 100, wherein the active mesa 181 directlyadjoins to one or two of the gate structures 150. In the active mesa 181the body zone 115 forms a first pn junction pn1 with a drift structure120 and a second pn junction pn2 with a source zone 110, wherein thesource zone 110 may be formed between the first surface 101 and the bodyzone 115.

The body zone 115 and the source zone 110 are electrically connected toa first load electrode 310, which may form or which may be electricallyconnected or coupled to a first load terminal L1. The drift structure120 is electrically connected or coupled directly or through a pnjunction to a second load electrode 320, which may form or which may beelectrically connected or coupled to a second load terminal L2.

The semiconductor device 500 further includes a heat sink structure 170extending from the first surface 101 into the drift structure 120.Either the specific thermal capacity of the heat sink structure 170 issignificantly greater than the specific thermal capacity of the gatestructure 150, or the specific thermal conductivity of the heat sinkstructure 170 is significantly greater than the specific thermalconductivity of the gate structure 150, or both the specific thermalcapacity and the specific thermal conductivity of the heat sinkstructure 170 are significantly greater than the specific thermalcapacity and the specific thermal conductivity of the gate structure150. For example, the specific thermal capacity of the heat sinkstructure 170 is at least twice or at least tenfold the specific thermalcapacity of the gate structure 150. Alternatively or in addition, thespecific thermal conductivity of the heat sink structure 170 is at leasttwice, at least five times, at least tenfold or at least twenty timesthe specific thermal conductivity of the gate structure 150.

According to an embodiment at least some of the heat sink structures 170are thermally connected to a cooling structure formed outside of and ina distance to the semiconductor body 100. The cooling structure may bethe first load electrode 310, the control electrode 330 or an auxiliaryelectrode electrically separated from the first load electrode 310 andthe control electrode 330.

The heat sink structure 170 absorbs, dissipates, or absorbs anddissipates thermal energy to a higher degree than the gate structures150. Since the heat sink structures 170 directly adjoin to electricactive portions of the semiconductor body 100, thermal energy generatedin a portion of the drift structure 120 next to the gate structures 150during an overcurrent condition, short-circuit or avalanche breakdowncan be instantly and directly removed from the semiconductor body 100without that the thermal energy passes further semiconductor regions atthe risk of thermal destruction of the crystal lattice of thesemiconductor body 100.

In IGBTs with high short-circuit ruggedness active gate structures aretypically formed only in a portion of the available total chip area.Further trench structures with the same dimensions as the gatestructures 150, e.g. field plate structures are typically formed inaddition to the gate structures 150, wherein the further trenchstructures and the gate structures form a regular pattern.

The same patterning process may be used to define both the gatestructures 150 and the heat sink structures 170 such that the heat sinkstructures 170 may be formed at comparatively low additional processcomplexity. A vertical extension of the heat sink structures 170 maydeviate from a vertical extension of the gate structures 150 by not morethan 20%, e.g., by not more than 10%, or at most 5%. According to anembodiment, the vertical extension of the heat sink structures 170 isequal to the vertical extension of the gate structures 150. The heatsink structures 170 may be stripe-shaped with a first horizontalextension exceeding at least ten times a second horizontal extensionorthogonal to the first horizontal extension or may be dot-shaped withboth horizontal extensions within the same order of magnitude, e.g.,approximately equal.

By configuring at least some of the further trench structures as heatsink structures 170, thermal ruggedness of the semiconductor device 500can be improved without any loss of area efficiency. Using the furthertrench structures for dissipating thermal energy allows for furtherincreasing the horizontal area ratio between trench structures on onehand and mesas on the other hand. The heat sink structures 170 directlyadjoin portions of the semiconductor body 100 in which otherwise thermalconductivity is low because of the patterning with the trenchstructures. Compared to heat dissipation through the gate structuresalone, heat can be dissipated through a significantly larger thermalcontact area. In addition, the heat sink structures 170 may contributeto hole confinement and in this way contribute to the reduction oflosses.

In FIG. 1B the semiconductor device 500 further includes field platestructures 160 extending from the first surface 101 into the driftstructure 120. The field plate structures 160 include a conductive fieldplate 165 and a field dielectric 161 separating the field plate 165 fromthe semiconductor body 100. The field plate 165 may be electricallyconnected to the first load electrode 310, to the control electrode 330or to a field plate terminal separated from the control terminal G andthe first load terminal L1.

The heat sink structure 170 may be electrically non-conductive, mayinclude an electrically conductive portion without electric connection,or may include an electrically conductive portion electrically connectedto a cooling body outside of the semiconductor body 100, e.g., the firstload electrode 310.

FIGS. 2A to 2E illustrate different embodiments of the heat sinkstructures 170 partly in combination with field plate structures 160 fora layout with four further trench structures formed between pairs ofactive gate structures 150. Similar considerations apply to embodimentswith one, two, three, five, six or more further trench structuresbetween each pair of gate structures 150.

FIG. 2A refers to a semiconductor device 500, which semiconductor body100 is based on a crystalline semiconductor material, e.g., Si, Ge,SiGe, SiC or an A_(III)B_(V) semiconductor. At a front side thesemiconductor body 100 has a first surface 101 which may be defined by aplane spanned by coplanar top surfaces of active and idle mesas 181, 182formed from sections of the semiconductor body 100 between neighboringtrench structures. A planar second surface 102 at the opposite rear sideis parallel to the first surface 101.

A minimum distance between the first and second surfaces 101, 102 isselected to achieve a specified voltage blocking capability of thesemiconductor device 500 and may be at least 20 μm. For example, thedistance between the first and second surfaces 101, 102 may be in arange from 90 μm to 110 μm for a semiconductor device 500 specified fora blocking voltage of about 1200 V. Embodiments related to semiconductordevices 500 with higher blocking capability may provide semiconductorbodies 100 with a thickness of several 100 μm.

In a plane perpendicular to the cross-sectional plane the semiconductorbody 100 may have a rectangular shape with an edge length in the rangeof several millimeters. A normal to the first surface 101 defines avertical direction and directions orthogonal to the vertical directionare horizontal directions.

In the semiconductor body 100 a pedestal layer 130 is sandwiched betweena drift structure 120 and the second surface 102. The pedestal layer 130may have the same conductivity type as the drift structure 120, acomplementary conductivity type, or may include zones of bothconductivity types. In the pedestal layer 130 a dopant concentrationalong the second surface 102 is sufficiently high to form an ohmiccontact with a metal layer formed in direct contact with the secondsurface 102.

The drift structure 120 may include a lightly doped drift zone 121 and amore heavily doped buffer or field stop zone 129 sandwiched between thedrift zone 121 and the pedestal layer 130. A dopant concentration in thedrift zone 121 may gradually or in steps increase or decrease withincreasing distance to the first surface 101 at least in portions of itsvertical extension. According to other embodiments the dopantconcentration in the drift zone 121 may be approximately uniform. A meandopant concentration in the drift zone 121 may be between 1E12 cm⁻³ and1E15 cm⁻³, for example in a range from 5E12 cm⁻³ to 8E13 cm⁻³. A meandopant concentration in the buffer or field stop zone 129 is at leastfive times, for example at least ten times as high as the mean dopantconcentration in the drift zone 121.

Gate structures 150 extend from the first surface 101 into thesemiconductor body 100. The gate structures 150 may include a conductivegate electrode 155 and a gate dielectric 151 separating the gateelectrode 155 from the semiconductor body 100. The gate electrode 155may be a homogeneous structure or may have a layered structure includingone or more metal containing layers. According to an embodiment, thegate electrode 155 may include or consist of a heavily dopedpolycrystalline silicon layer.

The gate dielectric 151 may include or consist of a semiconductor oxide,e.g., a thermally grown or deposited silicon oxide, a semiconductornitride, e.g., deposited or thermally grown silicon nitride, asemiconductor oxynitride, e.g., silicon oxynitride, by way of example.

The gate structures 150 may be stripe-shaped or dot-shaped. A verticalextension of the gate structures 150 may range from 1 μm to 30 μm, e.g.,from 3 μm to 7 μm. A lateral width of the active mesas 181 and the idlemesas 182 may range from 0.05 μm to 10 μm, e.g., from 0.15 μm to 1 μm. Adistance between the first surface 101 and the first pn junction pn1 mayrange from 0.5 μm to 5 μm, e.g., from 1 μm to 1.5 μm.

Between neighboring gate structures 150 one or more heat sink structures170 may extend from the first surface 101 into the drift structure 120.The heat sink structures 170 may be stripe-shaped or dot-shaped. Forexample, horizontal cross-sections of both the gate structures 150 andthe heat sink structures 170 may be stripe-shaped or both may bedot-shaped. According to an embodiment, the gate structures 150 arestripe-shaped and the heat sink structures 170 are dot-shaped or thegate structures 150 are dot-shaped and the heat sink structures 170 arestripe-shaped.

Semiconducting portions of transistor cells TC are formed in activemesas 181 that separate the gate structures 150 from neighboring trenchstructures, e.g., from neighboring heat sink structures 170. The activemesas 181 include body zones 115 and source zones 110, wherein the bodyzones 115 form first pn junctions pn1 with the drift structure 120 andsecond pn junctions pn2 with the source zones 110.

Portions of the semiconductor body 100 separating neighboring heat sinkstructures 170 form idle mesas 182. The idle mesas 182 may or may notinclude idle body zones 115 z and/or idle source zones 110 z withvertical extensions corresponding to the vertical extensions of thesource zones 110 and the body zones 115 in the active mesas 181.

An interlayer dielectric 210 may be formed on the first surface 101. Theinterlayer dielectric 210 may include one or more layers ofsemiconductor oxide, e.g., silicon oxide, semiconductor nitride, e.g.,silicon nitride, or semiconductor oxynitride, e.g., silicon oxynitride,which may be layers thermally grown on or deposited on the semiconductorbody 100, and/or one or more layers of doped or undoped glass, e.g., BSG(boron silicate glass), PSG (phosphorus silicate glass), BPSG (boronphosphorus silicate glass), FSG (fluorosilicate glass), USG (undopedsilicate glass) or a spin-on glass.

A first load electrode 310 is formed on the interlayer dielectric 210such that the interlayer dielectric 210 is sandwiched between the firstload electrode 310 and the semiconductor body 100. The first loadelectrode 310 may form or may be electrically coupled or connected to afirst load terminal L1, which may be an emitter terminal of an IGBT, asource terminal of an FET or an anode terminal of a power semiconductordiode, by way of example.

Source contact structures 315 a extend from the first load electrode 310through the interlayer dielectric 210 to or into the semiconductor body100 and electrically connect the source zones 110 and the body zones 115of the active mesas 181 with the first load electrode 310.

A second load electrode 320 directly adjoining the pedestal layer 130and the second surface 102 may form or may be electrically coupled orconnected to a second load terminal L2, which may be a collectorterminal of an IGBT, a drain terminal of an FET or a cathode terminal ofa power semiconductor diode, by way of example.

Each of the first and second load electrodes 310, 320 may consist of orcontain, as main constituent(s) aluminum (Al), copper (Cu), or alloys ofaluminum or copper, for example AlSi, AlCu or AlSiCu. According to otherembodiments, at least one of the first and second load electrodes 310,320 may contain, as main constituent(s), nickel (Ni), titanium (Ti),tungsten (W), tantalum (Ta), silver (Ag), gold (Au), platinum (Pt),and/or palladium (Pd). For example, at least one of the first and secondload electrodes 310, 320 may include two or more sub-layers, whereineach sub-layer contains one or more of Ni, Ti, Ag, Au, Pt, W, and Pd asmain constituent(s), e.g., a silicide, a nitride and/or an alloy.

The heat sink structures 170 may be dielectric or low-conductivestructures that may be homogeneous or layered, wherein dielectric orlow-conductive portions may separate a highly electric conductiveportion from the semiconductor body 100. At least one of the materialsof the heat sink structures 170 may have a significant higher specificthermal capacity than the gate electrode 155. For example, the heat sinkstructures 170 may contain an electrically non-conductive PCM (phasechange material), which may directly adjoin to the semiconductor body100 and which may embed thermally high-conductive inclusions such ascarbon nanotubes, diamond particles and/or graphene particles.

According to another embodiment the heat sink structures 170 may includea thermally high-conductive dielectric portion directly adjoining to thesemiconductor body 100 and an electrically conductive material with highthermal capacity, for example an electrically conductive PCM. Theinterlayer dielectric 210 may separate the heat sink structures 170 fromelectrically conductive structures formed at the front side of thesemiconductor body 100, for example from the first load electrode 310.

A width of the active and idle mesas 181, 182 may be in a range from0.1·L to 3·L, wherein L is the mean free path length of phonons in thesemiconductor body 100 at a temperature of 125° C.

In FIG. 2B the trench structures of the semiconductor device 500 includefield plate structures 160 extending from the first surface 101 into thedrift structure 120. The trench structures that form the gate structures150, the field plate structures 160 and the heat sink structures 170 mayhave the same vertical and lateral extensions and may form a regularstripe pattern with uniform center-to-center distance.

The field plate structures 160 include a conductive field plate 165 anda field dielectric 161 separating the field plate 165 from thesemiconductor body 100. The field plate 165 may be a homogeneousstructure or may be a layered structure including one or more metalcontaining layers. According to an embodiment the field plate 165 mayinclude or consist of a heavily doped polycrystalline silicon layer.Materials and internal configuration of the gate electrode 155 and thefield plate 165 may be the same.

The field dielectric 151 may include or consist of a semiconductoroxide, e.g., a thermally grown or deposited silicon oxide, asemiconductor nitride, e.g., a deposited or thermally grown siliconnitride, a semiconductor oxynitride, e.g., a silicon oxynitride, by wayof example. Materials and internal configuration of the field dielectric161 and the gate dielectric 151 may be the same. Field plate contactstructures 315 b may extend from the first load electrode 310 throughthe interlayer dielectric 210 to or into the field plates 165 and mayelectrically connect the field plates 165 with the first load electrode310.

The number of field plate structures 160 may be smaller than, greaterthan or equal to the number of heat sink structures 170. At least someof the field plate structures 160 may be formed next to active mesas181. According to another embodiment, the heat sink structures 170 maybe arranged between the field plate structures 160 and the active mesas181.

FIG. 20 shows heat sink structures 170 thermally connected to a coolingbody outside of the semiconductor body 100. According to the illustratedembodiment, heat contact structures 315 c extend from the first loadelectrode 310 through the interlayer dielectric 210 to or into the heatsink structures 170. The heat sink structures 170 may include thermallyhigh-conductive and electrically non-conductive materials, or may have alayered structure with a thermally high-conductive dielectric portion171 separating a thermally and electrically high-conductive centerportion 175 of the heat sink structures 170 from the semiconductor body100 as illustrated in FIG. 2D.

FIG. 2E refers to an embodiment with all trench structures except thegate structures 150 being effective as heat sink structures 170 with athermally high-conductive dielectric portion 171 separating anelectrically and thermally high-conductive center portion 175 from thesemiconductor body 100. Field plate contact structures 315 b may beeffective as the heat contact structures 315 c of FIGS. 2C and 2D andelectrically and thermally connect the center portions 175 with thefirst load electrode 310 such that the heat sink structures 170 areeffective as field plate structures.

The materials and the internal configuration of the gate electrode 155may differ from or may be the same as the material configuration of thecenter portions 175 of the heat sink structures 170. In addition or inthe alternative, the material configuration of the gate dielectric 151may differ or may be the same as that of the dielectric portion 171 ofthe heat sink structures 170.

FIGS. 3A to 3I refer to embodiments with a heat sink fill 173dissipating thermal energy within an outer contour of the semiconductorbody 100 by locally converting thermal energy into chemical or physicalbond energy, for example by changing the phase of a suitable materialfrom a low-energy state into a high-energy state. The phase change maybe a solid-to-solid phase change with a phase change temperature betweenthe nominal operation temperature and a critical temperature at whichthe crystal of the semiconductor body 100 is irreversibly destroyed.

The phase change may be reversible such that the PCM changes back intothe initial state when the surrounding semiconductor body 100 cools downto the temperature of the normal operation mode and the heat sink fill173 releases thermal energy back to the semiconductor body 100.According to other embodiments the phase change may be irreversible suchthat the semiconductor device 500 sustains only a certain number ofshort-circuits, over-current conditions and/or avalanche breakdowns.

In each of FIGS. 3A to 3I, both the gate structures 150 and the heatsink structures 170 may be stripe-shaped or dot-shaped. For example,horizontal cross-sections of both the gate structures 150 and the heatsink structures 170 may be stripe-shaped or both may be dot-shaped.According to an embodiment, the gate structures 150 are stripe-shapedand the heat sink structures 170 are dot-shaped or the gate structures150 are dot-shaped and the heat sink structures 170 are stripe-shaped.In case of dot-shaped cross-sections, FIGS. 3A to 3I showcross-sectional views in two orthogonal vertical planes.

The heat sink structures 170 of FIGS. 3A to 3I may be insulated. Forexample, portions of the interlayer dielectric 210 may cover the heatsink structures 170 and may separate the heat sink structures 170 fromconductive structures at the front side, e.g., the first load electrode310. According to other embodiments, the heat sink structures 170 ofFIGS. 3A to 3I may be connected to a cooling body outside of thesemiconductor body 100 through a thermally high-conductive connection,which may be electrically non-conductive. The cooling body may be one ofthe electrodes of the semiconductor device 500. For further details asregards FIGS. 3A to 3I, reference is made to the description of FIGS. 2Ato 2E.

In FIG. 3A a heat sink structure 170 includes a homogeneous heat sinkfill 173, which may be or contain a PCM. According to an embodiment, theheat sink fill 173 is an electrically non-conductive PCM. The heat sinkfill 173 locally dissipates thermal energy generated in the drift zone121, e.g., by reversibly changing the phase of a PCM portion of the heatsink fill 173.

In FIG. 3B the heat sink structure 170 includes a barrier layer 174separating the heat sink fill 173 from the semiconductor body 100. Thebarrier layer 174 is thermally highly conductive and may preventcontamination agents contained in the heat sink fill 173 from diffusinginto the semiconductor body 100. The barrier layer 174 may be or mayinclude a polycrystalline diamond layer deposited, e.g., by chemicalvapor deposition or an inorganic ceramic such as AlN, boron nitride orsilicon nitride Si₃N₄, by way of example. According to otherembodiments, the barrier layer 174 may be effective as a thermallyhigh-conductive shell for the heat sink fill 173.

The heat sink fill 173 may homogeneously fill the complete heat sinkstructure 170 or may only partially fill the heat sink structure 170.

In FIG. 3C the material of the heat sink fill 173 is deposited such thata central gap 175 a is formed in the center of the heat fill structure170.

In FIG. 3D the heat sink structure 170 includes air pockets (airinclusions) 175 b.

Voids in the heat sink fill 173, e.g., the gap 175 a or the air pockets175 b may be filled with a fluid or, for example ambient air, and mayreduce thermal-mechanical stress induced by the heat sink fill 173 intothe semiconductor body 100, e.g., when the heat sink fill 173 changesthe phase.

The heat sink fill 173, e.g., a PCM, may directly adjoin to thesemiconductor body 100. According to the embodiments of FIG. 3E to 3G athermally high-conductive structure may improve thermal conductionbetween the semiconductor body 100 and the heat sink fill 173, whereinthe thermally high-conductive structure may be a continuous layer or mayinclude separated portions formed along the interface to thesemiconductor body 100.

In FIG. 3E a continuous thermally high-conductive dielectric portion 171may separate the heat sink fill 173 from the semiconductor body 100. Thedielectric portion 171 may be a layer containing diamond or diamond-likematerials, boron nitride, aluminum nitride and/or beryllium oxide, byway of example.

FIG. 3F shows spatially separated carbon nanotubes 176 a extending froman interface between the heat sink structure 170 and the semiconductorbody 100 into the heat sink structure 170. The carbon nanotubes 176 amay be oriented perpendicular to the interface between the heat sinkstructure 170 and the semiconductor body 100. The heat sink fill 173 maybe a material with high thermal conductivity and/or high thermalcapacity, e.g., a PCM and may embed the carbon nanotubes 176 a and/ormay partially or completely fill the carbon nanotubes 176 a.

Formation of the carbon nanotubes 176 a may include formation ofcrystallization nuclei on sidewalls of trenches temporarily extendinginto the semiconductor body 100. The crystallization nuclei may containmetal atoms effective as crystallization catalyst, for example, iron(Fe), cobalt (Co) or nickel (Ni) atoms. Then the carbon nanotubes aregrown by CVD (chemical vapor deposition) using a carbon hydrogen gas,e.g., ethine as precursor material at a temperature of about 700° C. Aplasma may promote growth of the carbon nanotubes. The carbon nanotubes176 a drastically improve thermal conduction from the semiconductor body100 into the heat sink structure 170 and within the heat sink structure170.

In FIG. 3G isolated thermally high-conductive diamond crystallites 176 bmay be formed along the interface between the heat sink structure 170and the semiconductor body 100. A mean diameter of the diamondcrystallites 176 b may be between 50 nm and 2 μm, for example in a rangefrom 100 nm to 1 μm. By adapting the mean diameter of the diamondcrystallites 176 b to a specific layout, e.g., to a horizontal dimensionof the heat sink structure 170, the total thermal conductivity andresponse time of the heat sink structure 170 can be finely adjusted. Thediamond crystallites 176 b may be formed at temperatures in a range from500° C. to 1200° C. at a pressure below 760 Torr using carbon hydrogenprecursors, e.g., methane in presence of hydrogen radicals in excess.The hydrogen radicals may be formed in a plasma or at a heated tungsten(ON) wire. Deposition of the diamonds may be accelerated by addingdiamond seed crystallites or by generating lattice defects in thesemiconductor body 100.

Alternatively or in addition to thermally high-conductive structures 176a, 176 b formed along the interface between the heat sink structure 170and the semiconductor body 100, the heat sink structures 170 may containthermally high-conductive auxiliary structures embedded within the heatsink fill 173.

In FIG. 3H the heat sink fill 173 embeds diamond crystallites 177 a.

The heat sink structure 170 illustrated in FIG. 3I embeds thermallyhigh-conductive graphene flakes 177 b, which may be graphene leaves.

Composite heat sink structures 170 including PCM embedding diamondcrystallites or graphene leaves may be formed by a sol-gel process or byfilling temporary trenches in the semiconductor body 100 with solutionsor suspensions and then drying the solutions or suspensions in thetrenches. Heat sink structures 170 including an electricallyhigh-conductive heat sink fill 173 and a thermally high-conductive,continuous dielectric portion 171 may be used as field plate structures160 and/or as gate structures 150.

According to an embodiment, the heat sink fill 173 contains ceramic PCMparticles with a phase change from solid to solid at a temperaturebetween the normal operation temperature of the semiconductor device 500and a critical temperature at which the crystal lattice of thesemiconductor body 100 is irreversibly damaged. A size of the ceramicPCM particles may be in a range from some nanometers to somemicrometers. The ceramic PCM particles may embed, may be embedded in, ormay be attached or embedded to carbon nanotubes. According to anembodiment the PCM is germanium tellurium (GeTe) with a phase changetemperature between 350 and 400 degree Celsius.

The heat sink fill 173 may include core/shell structures with a shell ofthermally high-conductive material, e.g., an aluminum nitride ceramic.The shell encapsulates a PCM. A thin dielectric layer from a heatresistive polymer such as polyimide may coat and envelop the shell.

In FIGS. 4A to 4B, the heat sink structures 170 release the thermalenergy to a cooling body 340 outside of the semiconductor body 100,wherein thermal conductivity of the heat sink structures 170 is higherthan in the gate structures 150.

In FIG. 4A the heat sink structure 170 includes a dielectric portion 171from a thermally high-conductive material such as diamond, adiamond-like material, boron nitride, aluminum nitride and/or berylliumoxides. The dielectric portion 171 conducts thermal energy from thesemiconductor body 100 to an electrically high-conductive centralportion 175, which may be heavily doped polycrystalline silicon, e.g.,the same material as used as gate electrode or field plate. A heatcontact structure 315 c thermally connects the electricallyhigh-conductive central portion 175 with a cooling body 340 outside ofthe semiconductor body 100. The cooling body 340 may be one of theelectrodes of the semiconductor device 500 or a metallizationelectrically separated from the electrically active electrodes of thesemiconductor device 500. For example, the cooling body 340 may be aportion of the first load electrode 310.

Materials for the dielectric portion 171 may be used exclusively indedicated heat sink structures 170, or also as field dielectric in fieldplate structures, and/or also as gate dielectric in gate structures.

In addition to or alternatively to a thermally high-conductivedielectric portion 171, the heat sink structure 170 may include acentral portion 175 formed from silicon carbide (SiC) which has a higherthermal conductivity than polycrystalline silicon. During deposition ofsilicon carbide, the carbon content may be varied to increase thethermal conductivity. Silicon carbide may be used exclusively in theheat sink structures 170 or may also be used as field plate and/or asgate electrode.

The heat sink structure 170 of FIG. 4B includes a dielectric portion 171which is thinner than the gate dielectric 151 such that in the heat sinkstructure 170 the thermal resistance between the semiconductor body 100and the electrically high-conductive central portion 175 is lower thanbetween the semiconductor body 100 and the gate electrode 155. The heatsink structure 170 may be effective as field plate structures. Arelative permittivity of the dielectric portion 171 may be lower thanthat of silicon oxide such that at equal capacity the thermalconductivity of the dielectric portion 171 is lower than that of thegate dielectric 151.

FIG. 5A shows an n-IGFET 501 based on any of the previous embodiments.For the illustrated embodiments a first conductivity type is n-type anda second conductivity type is p-type. Similar considerations as outlinedbelow apply to embodiments with the first conductivity type being p-typeand the second conductivity type being n-type.

The source zones 110 and the pedestal layer 130 are heavily n-doped, thedrift zone 121 is lightly n-doped and the field stop layer 129 has a netdopant concentration of n-type dopants higher than in the drift zone 121and lower than in the pedestal layer 130. The body zones 115 are p-type.The first load electrode 310 forms or is electrically connected to asource terminal S and the second load electrode 320 forms or iselectrically connected to a drain terminal D A vertical extension of thegate structures 150 may be the same or may be smaller than a verticalextension of the field plate structures 160, which may be effective asheat sink structures 170.

FIG. 5B refers to an n-channel reverse blocking IGBT 502, The pedestallayer 130 is p-doped and the drift structure 120 may include a barrierlayer at least partially formed along the first pn junction pn1, whereina net n-type dopant concentration in the barrier layer 125 is higherthan in adjoining portions of the drift zone 121. A lateral width of atleast the gate structures 150 may have a maximum value in a distance tothe first surface 101. The first load electrode 310 forms or iselectrically connected to an emitter terminal E and the second loadelectrode 320 forms or is electrically connected to a collector terminalC.

FIG. 5C refers to an RC-IGBT 503 which differs from the reverse blockingIGBT of FIG. 5A mainly in that the pedestal layer 130 includes bothn-type first zones 131 and p-type second zones 132. In addition, furthercontact structures may electrically connect the idle mesas 182.

FIG. 6A shows a portion of a semiconductor device 500, which may be, forexample, a power semiconductor diode such as an MGD (MOS gated diode),an IGFET (insulated gate field effect transistor), e.g., a MOSFET (metaloxide semiconductor FET) in the usual meaning including FETs with metalgate and FETs with semiconductor gate, an IGBT (insulated gate bipolartransistor), e.g., a reverse blocking IGBT or a reverse conducting IGBT,or a device including further electronic circuits in addition to an MGD,IGFET, or IGBT functionality.

The semiconductor device 500 includes transistor cells TC, whereinsemiconducting portions of the transistor cells TC are formed along afront side of a semiconductor body 100, which is from a crystallinesemiconductor material such as silicon (Si), germanium (Ge), silicongermanium (Site) or an A_(III)B_(V) semiconductor. The semiconductingportion of a transistor cell includes a body zone forming a first pnjunction with a drift structure 120 and a second pn junction with asource zone.

A distance between a first surface 101 at the front side of thesemiconductor body 100 and an opposite second surface 102 at a rear sidemay be between 20 μm and several 100 μm. At the front side, sourcecontact structures 315 a form ohmic contacts between a first loadelectrode 310 and the source zones as well as between the first loadelectrode 310 and the body zones of the transistor cells TC, wherein thesource contact structures 315 a may directly adjoin the first surface101 or may extend into the semiconductor body 100.

At the rear side a second load electrode 320 may directly adjoin to thesecond surface 102. In the semiconductor body 100, a pedestal layer 130sandwiched between the drift structure 120 and the second load electrode320 may form an ohmic contact with the second load electrode 320.

Heat sink features 325 may extend from the second surface 102 at therear side into the semiconductor body 100. The heat sink features 325may be equally spaced or may be arranged denser in portions of thesemiconductor body 100 with lower thermal coupling to other heatconductive structures and/or with higher heat generation rate. The heatsink features 325 may be thermally and electrically connected to thesecond load electrode 320. A vertical extension of the heat sinkfeatures 325 may be at least 10% of the thickness of the semiconductorbody 100, e.g., about 20%, about 50%, or may be selected to approximatethe first pn junction pn1. According to another embodiment, the heatsink features 325 may extend from the second surface 102 to the firstsurface 101.

The heat sink features 325 may be parallel stripe structures that extendalong a horizontal direction parallel to the first surface 101.According to another embodiment, two orthogonal horizontal dimensions ofa heat sink feature 325 parallel to the first surface 101 may be withinthe same order of magnitude, e.g., approximately equal, wherein the heatsink features 325 may be arranged in orthogonal lines and rows. The heatsink features 325 may contain or consist of aluminum (Al), silver (Ag),copper (Cu), alloys of aluminum and/or copper, silicon carbide (SiC),heavily doped, e.g., co-doped polycrystalline silicon, aluminum oxide,aluminum nitride or a material embedding carbon nanotubes. The heat sinkfeatures 325 lead off thermal energy directly from where thermal energyis generated in the semiconductor body 100 in case of an overcurrentcondition or an avalanche breakdown. The heat sink features 325 improvethermal resilience of the semiconductor device 500 at low adverse impacton further device parameters.

The transistor cells TC may be planar transistor cells with gateelectrodes formed on the first surface 101 and outside of thesemiconductor body 100.

FIG. 6B refers to a semiconductor device 500 with trench structures 190that include gate structures and that extend from the first surface 101into the semiconductor body 100. Semiconducting portions of thetransistor cells TC are formed in portions of the semiconductor body 100directly adjoining to the trench structures 190. The heat sink features325 may be formed outside of a—with respect to the first surface101—vertical projection of the transistor cells TC and may be formed ata lateral distance to the transistor cells TC.

Heat sink features 325 may be formed between each pair of neighboringtrench structures 190 or in selected regions of the semiconductor body100, wherein in regions outside of the selected regions the transistorcells TC may directly adjoin to each other.

The second load electrode 320 and the heat sink features 325 may includethe same materials and may have the same material configuration or maybe based on different materials and/or different material combinations.

FIGS. 7A to 7C refer to an embodiment with equally spaced stripe-shapedheat sink features 325 extending from the rear side into a semiconductorbody 100 of a semiconductor device 500, which may be an MGT, an IGFET,or an IGBT by way of example. The semiconductor body 100 is based on acrystalline semiconductor material, e.g., Si, Ge, SiGe or anA_(III)B_(V) semiconductor. At a front side the semiconductor body 100has a first surface 101 which may be defined by a plane spanned bycoplanar top surfaces of mesas 180 formed from sections of thesemiconductor body 100 between neighboring trench structures 190. Aplanar second surface 102 at the opposite rear side is parallel to thefirst surface 101. Directions parallel to the first surface 101 arehorizontal directions and a direction perpendicular to the first surface101 is a vertical direction.

A minimum distance between the first and second surfaces 101, 102 isselected to achieve a specified voltage blocking capability of thesemiconductor device 500 and may be at least 15 μm. For example, thedistance between the first and second surfaces 101, 102 may be in arange from 20 μm to 60 μm.

In a plane perpendicular to the cross-sectional plane the semiconductorbody 100 may have a rectangular shape with an edge length in the rangeof several millimeters. A normal to the first surface 101 defines avertical direction and directions orthogonal to the vertical directionare horizontal directions.

In the semiconductor body 100 a pedestal layer 130 is sandwiched betweena drift structure 120 and the second surface 102. The pedestal layer 130may have the same conductivity type as the drift structure 120, acomplementary conductivity type, or may include zones of bothconductivity types. In the pedestal layer 130 a dopant concentrationalong the second surface 102 is sufficiently high to form an ohmiccontact with a metal layer formed in direct contact with the secondsurface 102.

The drift structure 120 may include a lightly doped drift zone 121 and amore heavily doped buffer or field stop zone 129 sandwiched between thedrift zone 121 and the pedestal layer 130. A dopant concentration in thedrift zone 121 may gradually or in steps increase or decrease withincreasing distance to the first surface 101 at least in portions of itsvertical extension. According to other embodiments the dopantconcentration in the drift zone 121 may be approximately uniform. A meandopant concentration in the drift zone 121 may be between 1E12 cm⁻³ and1E17 cm⁻³, for example in a range from 5E12 cm⁻³ to 5E16 cm⁻³. A meandopant concentration in the buffer or field stop zone 129 is at leastthree times, for example at least five times as high as the mean dopantconcentration in the drift zone 121.

Trench structures 190 extend from the first surface 101 into thesemiconductor body 100. A vertical extension of the trench structures190 may range from 300 nm to 15 μm, e.g., from 1 μm to 10 μm. A lateralwidth of the mesas 180 may range from 0.05 μm to 10 μm, e.g., from 0.15μm to 2 μm. The trench structures 190 may have the same verticalextension or may have different vertical extensions.

The trench structures 190 may include at least gate structures 150 thatinclude a conductive gate electrode 155 and a gate dielectric 151separating the gate electrode 155 from the semiconductor body 100. Thegate electrode 155 may be a homogeneous structure or may have a layeredstructure including one or more metal containing layers. According to anembodiment, the gate electrode 155 may include or consist of a heavilydoped polycrystalline silicon layer.

The gate dielectric 151 may include or consist of a semiconductor oxide,e.g., a thermally grown or deposited silicon oxide, a semiconductornitride, e.g., deposited or thermally grown silicon nitride, asemiconductor oxynitride, e.g., silicon oxynitride, by way of example.

The trench structures 190 may further include field plate structures 160between the gate structures 150 and a bottom of the trench structures190. The field plate structures 160 include a conductive field plate 165and a field dielectric 161 separating the field plate 165 from thesemiconductor body 100. The field plate 165 may be a homogeneousstructure or may be a layered structure including one or more metalcontaining layers. According to an embodiment the field plate 165 mayinclude or consist of a heavily doped polycrystalline silicon layer.Materials and internal configuration of the gate electrode 155 and thefield plate 165 may be the same. The field plate 165 may be electricallyconnected to one of the load electrodes of the semiconductor device 500,with the gate electrode 155 or with a field plate terminal electricallyseparated from the load terminals and the gate terminal.

The field dielectric 151 may include or consist of a semiconductoroxide, e.g., a thermally grown or deposited silicon oxide, asemiconductor nitride, e.g., a deposited or thermally grown siliconnitride, a semiconductor oxynitride, e.g., a silicon oxynitride, by wayof example. Materials and internal configuration of the field dielectric161 and the gate dielectric 151 may be the same.

Semiconducting portions of transistor cells TC are formed in the mesas180 that separate the trench structures 190 from each other. The mesas180 include body zones 115 and source zones 110, wherein the body zones115 form first pn junctions pn1 with the drift structure 120 and secondpn junctions pn2 with the source zones 110.

An interlayer dielectric 210 may be formed on the first surface 101. Theinterlayer dielectric 210 may include one or more layers ofsemiconductor oxide, e.g., silicon oxide, semiconductor nitride, e.g.,silicon nitride, or semiconductor oxynitride, e.g., silicon oxynitride,which may be layers thermally grown on or deposited on the semiconductorbody 100, and/or one or more layers of doped or undoped glass, e.g., BSG(boron silicate glass), PSG (phosphorus silicate glass), BPSG (boronphosphorus silicate glass), FSG (fluorosilicate glass), USG (undopedsilicate glass) or a spin-on glass.

A first load electrode 310 is formed on the interlayer dielectric 210such that the interlayer dielectric 210 is sandwiched between the firstload electrode 310 and the semiconductor body 100. The first loadelectrode 310 may form or may be electrically coupled or connected to afirst load terminal L1, which may be an emitter terminal of an IGBT, asource terminal of an FET or an anode terminal of a power semiconductordiode, by way of example.

Source contact structures 315 a extend from the first load electrode 310through the interlayer dielectric 210 to or into the semiconductor body100 and electrically connect the source zones 110 and the body zones 115in the mesas 180 with the first load electrode 310.

A second load electrode 320 directly adjoining the pedestal layer 130and the second surface 102 may form or may be electrically coupled orconnected to a second load terminal L2, which may be a collectorterminal of an IGBT, a drain terminal of an FET or a cathode terminal ofa power semiconductor diode, by way of example.

Each of the first and second load electrodes 310, 320 may consist of orcontain, as main constituent(s) aluminum (Al), copper (Cu), or alloys ofaluminum or copper, for example AlSi, AlCu or AlSiCu. According to otherembodiments, at least one of the first and second load electrodes 310,320 may contain, as main constituent(s), nickel (Ni), titanium (Ti),tungsten (W), tantalum (Ta), silver (Ag), gold (Au), platinum (Pt),and/or palladium (Pd). For example, at least one of the first and secondload electrodes 310, 320 may include two or more sub-layers, whereineach sub-layer contains one or more of Ni, Ti, Ag, Au, Pt, W, and Pd asmain constituent(s), e.g., a silicide, a nitride and/or an alloy.

Stripe-shaped heat sink features 325 extend from the rear side into thesemiconductor body 100. The heat sink features 325 may be dielectric orlow-conductive structures that may be homogeneous or layered, whereindielectric or low-conductive portions may separate a highly electricconductive portion from the semiconductor body 100. At least one of thematerials of the heat sink features 325 may have a significant higherspecific thermal capacity than the semiconductor body 100. For example,the heat sink features 325 may contain an electrically non-conductivePCM (phase change material), which may directly adjoin to thesemiconductor body 100 and which may embed thermally high-conductiveinclusions such as carbon nanotubes, diamond particles and/or grapheneparticles. For example, the heat sink features 325 may have any of theconfigurations described with reference to the heat sink structures 170of FIGS. 3A to 3I.

According to another embodiment, the heat sink features 325 may includeor consist of an electrically conductive portion, wherein the heat sinkfeatures 325 include a material with a higher thermal conductivity thanthe semiconductor body 100. For example, the material of the heat sinkfeatures 325 may be or may include silicon carbide, aluminum oxide,aluminum nitride or a material embedding carbon nanotubes. According toan embodiment, the material of the heat sink features 325 is heavilyp-doped, n-doped or co-doped polycrystalline silicon with a dopantconcentration greater 2E19 cm⁻³, e.g., greater than 3E19 cm⁻³ or greater6E19 cm⁻³. The material of the heat sink features 325 may directlyadjoin to the semiconductor body 100 or may be at least partiallyinsulated against the semiconductor body 100, e.g., by a dielectricportion with high thermal conductivity.

In the illustrated embodiment, all heat sink features 325 have the samevertical extension. According to other embodiments, the verticalextension of the heat sink features 325 may locally vary. The verticalextension of the heat sink features 325 may be selected such that theheat sink features 325 do not overlap with the trench structures 190along the vertical axis. In the illustrated embodiment, the heat sinkfeatures 325 overlap with the trench structures 190 and directly adjointo the field plate structures 160 in the trench structures 190.

The embodiment of FIGS. 7A to 7C refer to heat sink features 325completely formed from an electrically conductive material that directlyadjoins to portions of the drift zone 121 in the semiconductor body 100.

As illustrated in FIG. 7A, sub-regions including transistor cells TC andsub-regions for cooling may alternate along a horizontal directionorthogonal to a longitudinal axis of the stripe-shaped trench structures190.

Idle mesa sections 185 in the vertical projection of the heat sinkfeatures 325 may include an auxiliary portion 121 a of the drift zone121 directly adjoining to the first surface 101, Portions of the bodyzone 115 or an insulating structure may separate the auxiliary portions121 a of the drift zone 121 from neighboring source zones 110 in thehorizontal direction.

For example, for semiconductor devices 100 with a blocking voltage in arange from 20 V to 60 V a distance between neighboring heat sinkfeatures 325 or between heat sink features 325 and neighboring bodyzones 115 may be in a range from 1 μm to 4 μm. Distribution, dimensionsand materials of the heat sink features 325 may be selected to locallycompensate wafer bowing to some degree or to locally improve chargecarrier mobility.

The embodiment of FIGS. 8A to 8C refers to heat sink features 325extending from the second surface 102 up to the first surface 101. Theheat sink features 325 may include an electrically conductive portion325 a and a dielectric portion 325 b electrically separating theelectrically conductive portion 325 a horizontally from at leastsections of the semiconducting portions of the transistor cells TC,e.g., from the source zones 110 and the body zones 115 and the first pnjunction pn1. The material of the dielectric portion 325 b may have ahigher thermal conductivity than silicon oxide.

The gate dielectric 151 may include first portions 151 a electricallyseparating the gate electrode 155 from the body zones 115 and secondportions 151 b electrically separating the gate electrode 155 from theheat sink features 325. The second portions 151 b of the gate dielectric151 are significantly thicker than the first portions 151 a. A thicknessof the second portions 151 b may be approximately the same as athickness of the field dielectric 161, For further details reference ismade to the description of FIGS. 7A to 7C.

In FIGS. 9A to 9C the semiconductor device 500 includes column-like heatsink features 325 arranged in orthogonal lines and rows. The heat sinkfeatures 325 may be horizontally spaced from a vertical projection ofthe trench structures 190, For further details reference is made to thedescription of FIGS. 7A to 7C.

The semiconductor device 500 in FIGS. 10A to 100 includes stripe-shapedheat sink features 325 extending from the front side into the driftstructure 120 and including a dielectric portion 325 b electricallyseparating an electrically conductive portion 325 a horizontally from atleast sections of the body zones 115 close to the first pn junctions pn1and from portions of the drift structure 120.

The conductive portions 325 a or the complete heat sink features 325 areseparated from the second load electrode 320. Instead, the heat sinkfeatures 325 may be thermally and electrically connected with the firstload electrode 310 or with another metal structure at the front side.

According to another embodiment, the heat sink structures 325 may havehigher thermal capacity than the semiconductor body 100, wherein theheat sink structures 325 may be non-conductive or may include conductiveportions without electrical connection to any of the load electrodes310, 320 of the semiconductor device 500.

According to an embodiment the front side heat sink features 325 mayhave any of the configurations described with reference to the heat sinkstructures 170 of FIGS. 3A to 3I. For example, the heat sink features325 may contain an electrically non-conductive PCM, which may directlyadjoin to the semiconductor body 100 and which may embed thermallyhigh-conductive inclusions such as carbon nanotubes, diamond particlesand/or graphene particles.

Heat sink features 325 formed at the front side as described withreference to FIGS. 10A to 10C may be combined with heat sink features325 at the rear side as described with reference to FIGS. 7A to 7C andFIGS. 9A to 9C, wherein the front side and rear side heat sinkstructures may be formed as comb-like structures shifted to each othersuch that teeth of one comb-like structure are adjusted to gaps in theother comb-like structure and vice versa.

The arrangement of both front side heat sink structures and rear sideheat sink features 325 may be regular such that a population density ofthe heat sink features 325 is constant across a complete horizontalcross-sectional area of the semiconductor body 100.

According to the embodiment of FIGS. 11A to 11C, the distribution ofheat sink features 325 is adapted to an assumed thermal distribution inthe semiconductor body 100 without heat sink features 325. For example,in the absence of any heat sink features 325 in first sections 610 ofthe semiconductor body 100 along edges of a transistor cell area closeto a lateral outer surface 103 of the semiconductor body 100 or belowbond wires, a temperature of the semiconductor body 100 is lower than insecond sections 620 including central portions of the transistor cellarea in a distance to the bond wires. Then a population density of theheat sink features 325 in the second sections 620 is higher than in thefirst sections 610 to promote a better cooling of the hotter secondsections with reference to the cooler first sections and to achieve amore uniform temperature distribution in the semiconductor body 100.

The population density of the heat sink features 325 may graduallyincrease from the first sections 610 to the second sections intransition sections 615 between the first and second sections 610, 620.According to an embodiment, the heat sink features 325 have a lateraldimension of about 1 μm and are arranged at a distance of 4 μm to eachother in the second sections and at a distance of 10 μm to each other inthe first sections, wherein in the transition section 615 between thefirst and second sections 610, 620 the distance may gradually increasefrom 4 μm to 10 μm. For further details reference is made to thedescription of the previous Figures.

A method of manufacturing a semiconductor device with gate electrodesformed in trench structures extending from a front side into asemiconductor body and with heat sink features extending from a rearside into the semiconductor body may include the formation of front sidetrenches of different vertical extension at the front side of thesemiconductor body.

The semiconductor body may be thinned from the rear side to a bottomedge of the deepest front side trenches. A lithography process maydefine a hard mask on the rear side of the thinned semiconductor bodyfor defining the heat sink features. Using the hard mask, rear sidetrenches are etched into the semiconductor body. Then materials for theheat sink features may be deposited, e.g., a diffusion barrier forcopper atoms. The diffusion barrier may include at least one of atantalum layer, a tantalum nitride layer, a titanium layer, a titaniumnitride layer and a tungsten-containing layer. Then a fill material,e.g., copper Cu or tungsten W may be deposited to fill the rear sidetrenches.

According to an embodiment, a thin thermal oxide with a thickness ofabout 2 nm to 5 nm may be formed before deposition of the diffusionbarrier or in the alternative to deposition of the diffusion barrier inorder to improve the interface quality between the semiconductor bodyand the heat sink structure.

Although specific embodiments have been illustrated and describedherein, it will be appreciated by those of ordinary skill in the artthat a variety of alternate and/or equivalent implementations may besubstituted for the specific embodiments shown and described withoutdeparting from the scope of the present invention. This application isintended to cover any adaptations or variations of the specificembodiments discussed herein. Therefore, it is intended that thisinvention be limited only by the claims and the equivalents thereof.

What is claimed is:
 1. A semiconductor device comprising: a driftstructure formed in a semiconductor body, the drift structure formingfirst pn junctions with body zones of transistor cells; gate structuresextending from a first surface of the semiconductor body into the driftstructure; and heat sink structures extending from the first surfaceinto the drift structure, wherein a thermal capacity of the heat sinkstructures is greater than a thermal capacity of the gate structures. 2.The semiconductor device of claim 1, wherein a vertical extension of theheat sink structures is equal to or deviates by not more than 20% from avertical extension of the gate structures.
 3. The semiconductor deviceof claim 1, wherein the heat sink structures and the gate structuresform a regular stripe pattern with a same center-to-center distancebetween neighboring ones of the gate structures, between neighboringones of the heat sink structures, and between neighboring gate and heatsink structures.
 4. The semiconductor device of claim 3, whereinportions of the semiconductor body form (i) active mesas comprising thebody zones and directly adjoining to the gate structures and (ii) idlemesas between neighboring ones of the heat sink structures.
 5. Thesemiconductor device of claim 1, further comprising: field platestructures extending from the first surface into the drift structure. 6.The semiconductor device of claim 1, wherein the heat sink structurescomprise a dielectric portion with a specific thermal conductivityexceeding at least twice a specific thermal conductivity of a gatedielectric comprised in the gate structures.
 7. The semiconductor deviceof claim 1, wherein the heat sink structures comprise a dielectricportion thinner than a gate dielectric comprised in the gate structure.8. The semiconductor device of claim 1, wherein the heat sink structurescomprise an electrically conductive central portion and a dielectricportion separating the conductive portion and the semiconductor body. 9.The semiconductor device of claim 1, wherein the heat sink structurescomprise a material with a specific thermal capacitance exceeding atleast twice a specific thermal capacitance of a gate electrode comprisedin the gate structures.
 10. The semiconductor device of claim 1, whereinthe heat sink structures comprise a phase change material.
 11. Thesemiconductor device of claim 1, wherein the heat sink structurescomprise a gap or air pockets.
 12. The semiconductor device of claim 1,wherein the heat sink structures comprise thermally high-conductivestructures at an interface to the semiconductor body.
 13. Thesemiconductor device of claim 12, wherein the thermally high-conductivestructures comprise at least one of diamond crystallites, carbonnanotubes and graphene flakes.
 14. The semiconductor device of claim 12,wherein the heat sink structures comprise a heat sink fill containing atleast one of boron nitride and aluminum nitride.
 15. The semiconductordevice of claim 1, wherein the heat sink structures comprise ahigh-conductive central portion electrically separated from a gateelectrode comprised in the gate structure.
 16. The semiconductor deviceof claim 1, wherein the heat sink structures are configured as fieldplate structures.
 17. The semiconductor device of claim 1, furthercomprising: a source contact structure extending from the first surfaceinto the body zone wherein at least one of (i) a thermal conductivity ofthe heat sink structure is greater than a thermal conductivity of thesource contact structure; and (ii) a thermal capacity of the heat sinkstructure is greater than a thermal capacity of the source contactstructure.
 18. The semiconductor device of claim 1, wherein the heatsink structures are insulated from the body zone.
 19. The semiconductordevice of claim 1, wherein the gate structures are stripe-shaped with afirst horizontal extension exceeding at least ten times a secondhorizontal extension orthogonal to the first horizontal extension. 20.The semiconductor device of claim 1, wherein the gate structures aredot-shaped with a first horizontal extension and a second horizontalextension orthogonal to the first horizontal extension within the sameorder of magnitude.
 21. The semiconductor device of claim 1, wherein theheat sink structures are stripe-shaped with a first horizontal extensionexceeding at least ten times a second horizontal extension orthogonal tothe first horizontal extension.
 22. The semiconductor device of claim 1,wherein the heat sink structures are dot-shaped with a first horizontalextension and a second horizontal extension orthogonal to the firsthorizontal extension within the same order of magnitude.
 23. Asemiconductor device comprising: a drift structure formed in asemiconductor body, the drift structure forming a first pn junction withbody zones of transistor cells; gate structures extending from a firstsurface of the semiconductor body into the drift structure; and heatsink structures extending from the first surface into the driftstructure, wherein a thermal conductivity of the heat sink structures isgreater than a thermal conductivity of the gate structures, whereinportions of the semiconductor body form (i) active mesas comprising thebody zones directly adjoining to the gate structures and (ii) idle mesasbetween neighboring ones of the heat sink structures, and a width of theactive and idle mesas is in a range from 0.1 L to 3 L with L being amean free path length of phonons in the semiconductor body at 125° C.24. The semiconductor device of claim 23, wherein a vertical extensionof the heat sink structures is equal to or deviates by not more than 20%from a vertical extension of the gate structures.
 25. The semiconductordevice of claim 23, wherein the heat sink structures and the gatestructures form a regular stripe pattern with a same center-to-centerdistance between neighboring ones of the gate structures, neighboringones of the heat sink structures and neighboring gate and heat sinkstructures.
 26. The semiconductor device of claim 23, furthercomprising: field plate structures extending from the first surface intothe drift structure.
 27. The semiconductor device of claim 23, whereinthe heat sink structures include a material with a specific thermalconductivity exceeding at least twice a specific thermal conductivity ofa gate electrode comprised in the gate structure.
 28. The semiconductordevice of claim 23, wherein the heat sink structures include a centralportion of silicon carbide.
 29. The semiconductor device of claim 23,further comprising: a heat contact structure thermally connecting theheat sink structure with a cooling body outside of the semiconductorbody.
 30. The semiconductor device of claim 23, wherein the heat sinkstructures include a dielectric portion thinner than a gate dielectriccomprised in the gate structure.
 31. A semiconductor device comprising:a semiconductor body comprising semiconducting portions of a transistorcell; a source contact structure directly adjoining at least a sourcezone of the transistor cell at a front side of the semiconductor body;and heat sink features extending from a rear side into the semiconductorbody, wherein at least one of (i) a thermal conductivity of the heatsink features is greater than a thermal conductivity of thesemiconductor body; and (ii) a thermal capacity of the heat sinkfeatures is greater than a thermal capacity of the semiconductor body,wherein the heat sink features are arranged more densely in a centralsection of a transistor cell array than outside of the central sectionof the transistor cell array.
 32. The semiconductor device of claim 31,further comprising: trench structures extending from a first surface atthe front side into the semiconductor body, the trench structurescomprising a gate electrode and a gate dielectric separating the gateelectrode from the semiconductor body, wherein portions of thesemiconductor body between neighboring trench structures form mesas. 33.The semiconductor device of claim 32, wherein the heat sink featuresextend from a second surface opposite to the first surface into sectionsof the mesas oriented to the second surface.
 34. The semiconductordevice of claim 32, wherein the source zones are formed in portions ofthe mesas outside of a vertical projection of the heat sink feature, thevertical projection oriented orthogonal to the first surface.
 35. Thesemiconductor device of claim 32, wherein the heat sink features and thetrench structures are stripe-shaped structures and the heat sinkfeatures run orthogonal to the trench structures.
 36. The semiconductordevice of claim 31, further comprising: further heat sink featuresextending from the front side into the semiconductor body.
 37. Thesemiconductor device of claim 31, comprising: a plurality of the heatsink features at the rear side.
 38. The semiconductor device of claim31, wherein the heat sink features comprise an electrically conductiveportion from at least one material selected from copper, siliconcarbide, a material embedding carbon nanotubes, aluminum oxide, aluminumnitride and heavily doped polycrystalline silicon with a dopantconcentration of at least 2E19 cm⁻³.